As an energy storage device such as a secondary battery, there has been known an energy storage device which includes an electrode assembly and a case housing the electrode assembly. For example, patent document 1 describes an energy storage device which includes: a can and a lid forming a case; and an external terminal mounted on the lid and functioning as an electrode terminal. In the above-mentioned energy storage device, the external terminal includes: a base portion formed of a planar portion; and a penetration portion formed on the base portion and penetrating the lid. Further, in the above-mentioned energy storage device, an external insulator functioning as an insulating member is disposed between the base portion and the lid. The external insulator has a covering wall, and the covering wall covers side surfaces of the base portion of the external terminal disposed inside the covering wall.